1. Field of the Invention
The present invention relates to a radiation imaging apparatus for detecting an image based on a phase shift of radiation caused by a subject and an image processing method for use in a radiation imaging apparatus.
2. Description Related to the Prior Art
Radiation, for example, X-rays are attenuated depending on weight (atomic number) of an element constituting the substance, and density and thickness of the substance. By taking advantage of the characteristics of this attenuation, the X-rays are used as a probe for examining the inside of a subject in medical diagnoses and non-destructive inspections.
A common X-ray imaging apparatus is provided with an X-ray source for emitting the X-rays and an X-ray image detector for detecting the X-rays. A subject is placed between the X-ray source and the X-ray image detector. The X-rays emitted from the X-ray source are attenuated or absorbed by the subject and then are incident on the X-ray image detector. As a result, the image reflecting the intensity changes of the X-rays caused by the subject is detected using the X-ray image detector.
The X-ray absorption performance of a substance decreases as the atomic number of the element constituting the substance decreases. This causes a problem that a sufficient contrast cannot be obtained in the X-ray absorption image of living soft tissue or soft materials. For example, a cartilage portion constituting a joint of a human body and synovial fluid surrounding the cartilage portion are mainly composed of water, so that there is little difference between their amounts of X-ray absorption, resulting in little difference in contrast.
Recently, X-ray phase imaging has been studied actively to solve the above problem. The X-ray phase imaging is used to obtain an image (hereafter referred to as the phase contrast image) based on a phase shift, instead of an intensity change, of the X-rays caused by the subject through which the X-ray passes. The X-ray phase imaging is a method to image the phase shift of the X-rays based on the fact that the phase shift of the X-rays is larger in magnitude than the intensity change of the X-rays when the X-rays are incident on the subject. Thereby, a high contrast image is obtained even if the subject is composed of components with little difference in X-ray absorptivity. An X-ray imaging apparatus using an X-ray Talbot interferometer to detect the phase shift of the X-rays is known as one type of X-ray phase imaging apparatus. The X-ray Talbot interferometer uses two diffraction gratings and an X-ray image detector (see, for example, U.S. Pat. No. 7,180,979 (corresponding to WO2004/058070).
In the X-ray imaging apparatus, when viewed from the X-ray source, a first diffraction grating is disposed behind the subject. A second diffraction grating is placed downstream of the first diffraction grating by a Talbot length. The X-ray image detector is disposed behind the second diffraction grating. The Talbot length is a distance between the first diffraction grating and a position at which the X-rays passed through the first diffraction grating form a self image (fringe image) of the first diffraction grating due to the Talbot effect. The Talbot length is determined by a grating pitch of the first diffraction grating and an X-ray wavelength. The self image is modulated by refraction of the X-rays due to the phase shift caused by the subject. An image representing the phase shift is produced by detecting an amount of modulation.
A fringe scanning method is known as a method to detect the amount of modulation. In the fringe scanning method, the second diffraction grating is translationally moved (scanned), relative to the first diffraction grating, at a predetermined scanning pitch in a direction parallel to the first diffraction grating and vertical to a grating line (grid line) of the first diffraction grating. Every time the second diffraction grating is moved, the X-ray source emits the X-rays and the X-ray detector images the X-rays passed through the subject and the first and second diffraction gratings. A phase shift value (a phase difference from an initial position in the absence of the subject) is calculated from an intensity modulation signal that represents changes in pixel value of each pixel obtained with the X-ray image detector during the scanning. Thereby, an image related to the amount of modulation is produced. The image, referred to as the differential phase image, reflects a refraction index of the subject, and corresponds to a differential value of the phase shift of the X-rays.
As disclosed in the U.S. Patent No. 7,180,979, the phase shift value is calculated using a function (arg[ . . . ]) to extract an argument of a complex number or an arctangent function (tan−1[ . . . ]). Accordingly, the differential phase image is represented by values wrapped into a range (of−π to π, or of−π/2 to +π/2) of the function. The “wrapped” differential phase image may have a phase discontinuity at a data point where the value changes from the upper limit to the lower limit or from the lower limit to the upper limit of the range. A phase unwrapping process is carried out to eliminate the phase discontinuity and make values change smoothly (for example, see Japanese Patent Laid-Open Publication No. 2011-045655).
The phase unwrapping process starts from a starting point in the differential phase image and is carried out sequentially along a path. When the phase discontinuity is detected in the path, a value corresponding to the range of the above-described function is added to or subtracted from each data on and after to the phase discontinuity. Thereby, the phase discontinuity is eliminated, making the data continuous.
When the subject includes a body part having high X-ray absorption characteristics, for example, a bony part, the body part significantly attenuates the X-rays. This reduces intensity and amplitude of the intensity modulation signal. The calculation accuracy of the phase shift value is reduced in a region with the body part having the high X-ray absorption characteristics, which often results in phase unwrapping error. The phase unwrapping error occurs when the phase unwrapping process is performed on a normal data point mistakenly detected as the phase discontinuity. The phase unwrapping error also occurs when the phase discontinuity is mistakenly detected as the normal data point and the phase unwrapping process is not performed.
For example, when the bony part is on the path of the phase unwrapping process and causes the phase unwrapping error, an error value (corresponding to the range of the above-described function) is added to the subsequent data on the path. This causes streak noise in the differential phase image in a direction of the path of the phase unwrapping process. When the streak noise overlaps with soft tissue, for example, the cartilage portion, the streak noise hinders imaging of the soft tissue, being a region of interest in the X-ray phase imaging.